Local files won't sync to iPhone

[txbi89]

Plan

Premium

Country

UK 

Device

Laptop

Operating System

Windows 10

My Question or Issue

I have loads of local files that I use on my laptop and usually syncing them to my iPhone is no issue but this one file has messed everything up. I've tried everything but nothing has worked. It's still greyed out. To make matters worse, someone on this website told me to re-download the playlist, which got rid of every local file I have. I'm at my wits end. Does anyone know what to do? When I download the playlist on my phone, it says it's been downloaded but the songs are still grey.